1. Field of the Invention
The present invention generally relates to color diffusion transfer photography, and, more particularly, to photographic materials adapted for forming color transfer images by a color diffusion transfer photographic process.
2. Description of the Prior Art
A color diffusion transfer photographic material is generally composed of (1) a photosensitive element having three kinds of units (corresponding to the three principle colors) each comprising a combination of a light-sensitive silver halide emulsion layer and a dye image providing material associated therewith for providing a diffusible dye, (2) a processing element containing a liquid processing composition (an alkaline aqueous solution or aqueous dispersion) capable of developing the exposed photosensitive element and diffusing a diffusible dye formed from the dye image providing material, and (3) an image receiving element including an image receiving layer capable of image-wise receiving the diffused dye.
One of the features of a diffusion transfer color photographic material is that a natural color photographic print is obtained immediately after photography, and to utilize this feature effectively, an "instant photographic processing" mechanism which does not require stabilization processings (such as washing and fixing of color images after development), which is quite different from conventional color photography, is associated with the photographic material.
Since in color diffusion transfer photographic process a washing step and a fixing step are not employed, it is necessary, to achieve stable color images, to provide a means in the color photographic material for automatically stabilizing the color images.
As one stabilizing means for color images, it is known to place a neutralizing means in the color photographic material. That is, by employing a neutralizing means in the photographic material, alkaline components and salt forming reagents in a processing composition are absorbed therein after development to remove factors which destroy or change the color images formed.
Various means have been proposed to achieve the above as described in, for example, U.S. Pat. Nos. 3,362,819, 3,575,701 and 3,455,686. These attempts fundamentally employ a system where an acid polymer layer is formed on a support and a neutralization rate controlling layer (this layer is hereinafter referred to as a "timing layer") is formed on the acid polymer layer. In this system, the acid polymer layer acts to absorb the alkaline components and salt forming reagents in the processing composition while the timing layer acts to delay the start of the action of the acid polymer layer to prevent insufficient development caused by too fast an action of the acid polymer layer.
In addition, as materials for the timing layer, inert polymers or copolymers such as polyvinyl alcohol, partial acetalized products of polyvinyl alcohol, copolymers of vinyl acetate and vinyl alcohol, polyvinyl acetate (homopolymer), and gelatin, as well as a polyvinyl amide graft copolymers, have been used. The neutralizing means using such a polymer or copolymer shows the characteristic that the amounts of water, alkaline components, and salt forming reagents passing through the timing layer during development are passing at almost the same rate, and, as a result thereof, the pH in the image receiving layer and the photosensitive layer is quickly reduced.
If the pH of these layers is quickly reduced, it becomes difficult to sufficiently maintain the necessary pH for development, which results in making it difficult to provide a fast development rate as is required for practical purposes. On the other hand, if the pH is slowly reduced, it takes a comparatively long time to reach a stable pH region (i.e., 5 to 7, preferably 6 to 7) for the color images, and, thus, the color images tend to be stained (image stability is lost).